The objective of this research proposal is to further investigate the role of the Src kinase family member Fyn as a novel element of the nutrient-sensor system that regulates cellular and whole body energy homeostasis via AMPK and its upstream kinase, LKB1. In our published work (Bastie et. al. 2007 Cell Metab.), we have shown that the Fyn null animals present a marked reduction in adiposity, increased insulin sensitivity and increased AMPK activity coupled with increased fatty acid oxidation and energy expenditure. In the last 10 months, we have successfully demonstrated that LKB1, the main upstream kinase of AMPK in peripheral tissues, is a direct substrate for Fyn kinase. Moreover, LKB1 sub-cellular localization is regulated by tyrosine phosphorylation on Y261 and Y365 of LKB1 by Fyn kinase. Site directed mutagenesis of Y261 and Y365 results in re-localization of LKB1 from the nucleus into the cytoplasm, with a resultant increase in AMPK phosphorylation. These findings are consistent with the observed phenotype of the Fyn null mice. Importantly, the positive metabolic effects observed in the Fyn null mice (decreased adiposity and increased energy expenditure) are reproduced by acute pharmacological inhibition of Fyn kinase activity, resulting in weight loss via specific decrease of adiposity with no alteration of lean mass (Yamada, E., Pessin, J.E., Kurland, IJ., Schwartz, GJ., and Bastie, CC. Cell Metab. Under revision). These data place Fyn kinase and LKB1 as key potential therapeutic targets in obesity, a condition with a significant clinical unmet need. Based upon these data, we propose to further examine the novel functions of Fyn kinase as a fuel sensor. We will determine the precise mechanism (s) of Fyn kinase-dependent regulation of LKB1 using molecular and cellular tools. We will also explore the basis for upstream signals regulating Fyn kinase activity during the fed and fasted states and how this regulates LKB1 localization and subsequently AMPK activity. Finally, we will study the metabolic effects and the signaling pathways of pharmacological inhibition of Fyn function in animal models of obesity. PUBLIC HEALTH RELEVANCE: Obesity and diabetes are associated with dysfunctions of glucose and lipid metabolism. Lack of Fyn kinase increases insulin sensitivity, up-regulates lipid utilization and pharmacological inhibition of Fyn promotes weight loss. Therefore, Fyn kinase is a potential target for the development of specific drugs against insulin resistance associated with obesity and type 2 diabetes.